Testing subterranean water for a hazardous waste material repository

ABSTRACT

Techniques for testing subterranean water for one or more radioactive isotopes for a hazardous waste material repository include collecting, from a test drillhole formed from a terranean surface to a subterranean formation, a subterranean water sample from the subterranean formation; determining, with an accelerator mass spectrometry (AMS) system, a concentration of a radioactive isotope of an element in the subterranean water sample relative to a stable isotope of the element in the subterranean water sample; comparing the determined concentration of the radioactive isotope of the element in the subterranean water sample with a concentration of the radioactive isotope of the element in a surface water sample relative to the stable isotope of the element in the surface water sample; and based on the determined concentration of the radioactive isotope in the subterranean water sample being a specified percentage of the concentration of the radioactive isotope in the surface water sample, determining that the subterranean formation is a hazardous waste storage repository.

TECHNICAL FIELD

This disclosure relates to testing subterranean water and, moreparticular, testing subterranean water for one or more radioactiveisotopes for a hazardous waste material repository.

BACKGROUND

Storing hazardous waste material underground may have significant risks.One risk may be that the hazardous waste material, or byproducts of thehazardous waste material, may enter into a source of human-consumablewater. Some subterranean formations allow mobile water; that is themovement of water stored in the formation to a location in whichhuman-consumable water is located. Therefore, any hazardous wastematerial stored underground must be kept from a source of mobile water.

SUMMARY

In a general implementation, a method includes collecting, from a testdrillhole formed from a terranean surface to a subterranean formation, asubterranean water sample from the subterranean formation; determining,with an accelerator mass spectrometry (AMS) system, a concentration of aradioactive isotope of an element in the subterranean water samplerelative to a stable isotope of the element in the subterranean watersample; comparing the determined concentration of the radioactiveisotope of the element in the subterranean water sample with aconcentration of the radioactive isotope of the element in a surfacewater sample relative to the stable isotope of the element in thesurface water sample; and based on the determined concentration of theradioactive isotope in the subterranean water sample being a specifiedpercentage of the concentration of the radioactive isotope in thesurface water sample, determining that the subterranean formationincludes a hazardous waste storage repository.

In an aspect combinable with the general implementation, the radioactiveisotope is carbon-14 (¹⁴C) and the stable isotope is ¹²C or ¹³C; theradioactive isotope is chlorine-36 (³⁶Cl) and the stable isotope is³⁵Cl; the radioactive isotope is iodine-129 (¹²⁹I) and the stableisotope is ¹²⁷I; the radioactive isotope is beryllium-10 (¹⁰Be) and thestable isotope is ⁹Be; or the radioactive isotope is aluminum-26 (²⁶Al)and the stable isotope is ²⁷Al.

In an aspect combinable with any of the previous aspects, the specifiedpercentage is less than 50 percent.

An aspect combinable with any of the previous aspects further includescollecting the surface water sample from a surface water source.

In an aspect combinable with any of the previous aspects, the surfacewater source includes at least one of an aquifer or a water source atthe terranean surface in contact with the earth's atmosphere.

In an aspect combinable with any of the previous aspects, the surfacewater sample includes potable water.

In an aspect combinable with any of the previous aspects, collecting thesubterranean water sample from the subterranean formation includesoperating a downhole tool in the test drillhole to collect a core samplefrom the subterranean formation; retrieving the core sample to theterranean surface; and removing water from the core sample, the removedwater including the subterranean water sample.

In an aspect combinable with any of the previous aspects, collecting thesubterranean water sample from the subterranean formation is performedprior in time to collecting the subterranean water sample from thesubterranean formation.

In an aspect combinable with any of the previous aspects, thesubterranean formation includes a shale formation.

In an aspect combinable with any of the previous aspects, thesubterranean formation includes a permeability of less than about 0.01millidarcys.

An aspect combinable with any of the previous aspects further includesforming the test drillhole from the terranean surface to thesubterranean formation.

In an aspect combinable with any of the previous aspects, the testdrillhole includes a vertical drillhole.

In an aspect combinable with any of the previous aspects, thesubterranean formation includes a brittleness of less than about 10 MPa,where brittleness includes a ratio of compressive stress of thesubterranean formation to tensile strength of the subterraneanformation.

In an aspect combinable with any of the previous aspects, thesubterranean formation includes about 20 to 40% weight by volume of clayor organic matter.

In an aspect combinable with any of the previous aspects, thesubterranean formation includes an impermeable layer.

In an aspect combinable with any of the previous aspects, thesubterranean formation includes a leakage barrier defined by a timeconstant for leakage of a hazardous waste material of 10,000 years ormore.

In an aspect combinable with any of the previous aspects, thesubterranean formation includes a hydrocarbon or carbon dioxide bearingformation.

An aspect combinable with any of the previous aspects further includesinitiating creation of the hazardous waste storage repository in orunder the subterranean formation.

In an aspect combinable with any of the previous aspects, initiatingcreation of the hazardous waste storage repository in or under thesubterranean formation includes forming an access drillhole from theterranean surface toward the subterranean formation; and forming astorage drillhole coupled to the access drillhole in or under thesubterranean formation, the storage drillhole including a hazardouswaste storage area.

In an aspect combinable with any of the previous aspects, the accessdrillhole includes a vertical drillhole.

In an aspect combinable with any of the previous aspects, the accessdrillhole is the test drillhole.

In an aspect combinable with any of the previous aspects, the storagedrillhole includes a curved portion and a horizontal portion.

In an aspect combinable with any of the previous aspects, thesubterranean formation includes a thickness proximate the hazardouswaste material storage area of at least about 200 feet.

In an aspect combinable with any of the previous aspects, thesubterranean formation includes a thickness proximate the hazardouswaste material storage area that inhibits diffusion of a hazardous wastematerial through the subterranean formation for an amount of time thatis based on a half-life of the hazardous waste material.

An aspect combinable with any of the previous aspects further includesinstalling a casing in the access drillhole and the storage drillholethat extends from at or proximate the terranean surface, through theaccess drillhole and the storage drillhole, and into the hazardous wastematerial storage area of the storage drillhole.

An aspect combinable with any of the previous aspects further includescementing the casing to the access drillhole and the storage drillhole.

An aspect combinable with any of the previous aspects further includes,subsequent to forming the access drillhole, producing hydrocarbon fluidfrom the subterranean formation, through the access drillhole, and tothe terranean surface.

An aspect combinable with any of the previous aspects further includesstoring hazardous waste material in the hazardous waste storage area.

In an aspect combinable with any of the previous aspects, storinghazardous waste material in the hazardous waste storage area includesmoving a storage canister through an entry of the access drillhole thatextends into the terranean surface, the entry at least proximate theterranean surface, the storage canister including an inner cavity sizedto enclose the hazardous waste material; moving the storage canisterthrough the access drillhole and into the storage drillhole; and movingthe storage canister through the storage drillhole to the hazardouswaste storage area.

An aspect combinable with any of the previous aspects further includesforming a seal in at least one of the access drillhole or the storagedrillhole that isolates the hazardous waste storage area from the entryof the access drillhole.

In an aspect combinable with any of the previous aspects, the hazardouswaste material includes spent nuclear fuel or other radioactivematerial.

In an aspect combinable with any of the previous aspects, the storagecanister includes a connecting portion configured to couple to at leastone of a downhole tool string or another storage canister.

An aspect combinable with any of the previous aspects further includesmonitoring the hazardous waste material stored in the hazardous wastematerial storage area of the storage drillhole.

In an aspect combinable with any of the previous aspects, monitoring thehazardous waste material stored in the hazardous waste material storagearea of the storage drillhole includes removing the seal; and retrievingthe storage canister from the hazardous waste material storage area tothe terranean surface.

In an aspect combinable with any of the previous aspects, monitoring thehazardous waste material stored in the hazardous waste material storagearea of the storage drillhole includes monitoring at least one variableassociated with the storage canister from a sensor positioned proximatethe hazardous waste material storage area; and recording the monitoredvariable at the terranean surface.

In an aspect combinable with any of the previous aspects, the monitoredvariable includes at least one of radiation level, temperature,pressure, presence of oxygen, presence of water vapor, presence ofliquid water, acidity, or seismic activity.

An aspect combinable with any of the previous aspects further includes,based on the monitored variable exceeding a threshold value removing theseal; and retrieving the storage canister from the hazardous wastematerial storage drillhole portion to the terranean surface.

Implementations of subterranean water testing systems and methodsaccording to the present disclosure may also include one or more of thefollowing features. For example, subterranean water testing systems andmethods according to the present disclosure may be used to identify ordetermine that a particular subterranean formation is suitable as ahazardous waste material repository. The determined hazardous wastematerial repository may be used to store hazardous waste material, suchas spent nuclear fuel, isolated from human-consumable water sources. Thedetermined hazardous waste material repository may be suitable forstoring the hazardous waste material for durations of time up to, forexample, 1,000,000 years. As another example, subterranean water testingsystems and methods according to the present disclosure may confirm thata particular geologic formation is suitable as a hazardous wastematerial repository.

The details of one or more implementations of the subject matterdescribed in this disclosure are set forth in the accompanying drawingsand the description below. Other features, aspects, and advantages ofthe subject matter will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an example implementation of asubterranean water testing system according to the present disclosure.

FIG. 2A is a schematic illustration of an example implementation of ahazardous waste material storage repository system during a deposit orretrieval operation according to the present disclosure.

FIG. 2B is a schematic illustration of an example implementation of ahazardous waste material storage repository system during storage ofhazardous waste material.

FIG. 3 is a flowchart that illustrates an example process for testingsubterranean water for a ratio of a radioactive isotope of a particularelement with respect to a stable isotope of the particular element.

FIG. 4 is a flowchart that illustrates an example process for storinghazardous waste material in a subterranean formation from which waterhas been tested for a radioactive isotope concentration percentage.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of an example implementation of asubterranean water testing system 100. As shown in this example, thesystem 100 includes a test drillhole 104 formed from a terranean surface102, through a surface water formation 106, and into and throughsubterranean formations 108 and 110 that are deeper than the surfacewater formation 106. Each of the formations 106, 108, and 110 maycomprise a geologic formation formed of one or more rock types, as wellas water (e.g., fresh or brine) and in some cases other fluids (e.g.,hydrocarbon fluids). In this example, the test drillhole 104 is shown asa vertical drillhole. However, in alternative implementations, adirectional drillhole 124 (shown in dashed line) may be formed and usedin the system 100 in place of (or in addition to) the test drillhole104) according to the present disclosure.

The surface water formation 106, in this example, is a geologic layercomprised of one or more layered rock formations and includes one ormore surface water sources. For example, surface water formation 106 mayinclude one or more open water sources 116 (e.g., lakes, ponds, rivers,creeks). In some aspects, open water sources 116 are water sources thathave direct contact with the atmosphere 101. Surface water formation 106may also include one or more aquifers 118 that are not in direct contactwith the atmosphere 101 but are suitable for human consumption (e.g.,with or without conventional water treatment). Thus, in this exampleimplementation of system 100, surface water includes both open watersources 116 and aquifers 118. Examples of rock formations of which thesurface water formation 106 may be composed include porous sandstonesand limestones, among other formations.

Below the surface water formation 106, in this example implementation,are subterranean formations 108 and 110. One or both of the subterraneanformations 108 or 110 may include or hold subterranean water.Subterranean water, in this example system, is water that is not an openwater source or aquifer and is not in present-day contact with theatmosphere 101 (but may have been at some time in the past). In someaspects, subterranean water is non-potable or is not fit for humanconsumption (or both). System 100 may be used (e.g., as described withreference to FIG. 3 and process 300) to test one or both of subterraneanformations 108 or 110 for hazardous waste material storage according tothe subterranean water found in such formations.

System 100 also includes a downhole tool 112 (e.g., a core drill) thatcan be conveyed into the test drillhole 104 and to one or all offormations 106, 108, and 110 to procure a core sample 114 or core sample120. In this example, core sample 114 include subterranean water whilecore sample 120 includes surface water. Thus, a subterranean watersample may be obtained from core sample 114, while a surface watersample may be obtained from core sample 120 (or open water source 116 oraquifer 118). Although core sample 114 is shown as being obtained fromsubterranean formation 110, one or more core samples 114 may be obtainedfrom this formation or subterranean formation 108 (or both).

System 100 also includes an accelerator mass spectrometry system (AMS)122. The AMS system 122, generally, may be operated to perform manytesting functions. For example, the AMS system 122 may analyzesubstances, such as water, to detect naturally occurring, long-livedradio-isotopes (of elements) such as beryllium-10 (¹⁰Be), chlorine-36(³⁶Cl), aluminum-26 (²⁶Al), iodine-129 (¹²⁹I), and carbon-14 (i.e.,radiocarbon or ¹⁴C) in such substances. In some cases, certainradioactive isotopes, such as ³⁶Cl and ¹²⁹I, may be produced in theatmosphere 101 by cosmic radiation, and mixed with surface water, or isproduced directly in the surface water or surface rock. Thus, substancessuch as surface water sources may have a particular concentration ofsuch radioactive isotopes of the elements based on time period of theatmosphere 101 to which the substances have been exposed. Substances nolonger exposed to the atmosphere 101, such as subterranean water,experience a decay in the concentration of such radioactive isotopes(e.g., ³⁶Cl relative to the concentration of the stable isotope, ³⁷Cl,of the element chlorine; ¹²⁹I relative to the concentration of thestable isotope, ¹²⁷I, of the element iodine; ¹⁰Be relative to theconcentration of the stable isotope, ⁹Be, of the element beryllium; ¹⁴Crelative to the concentration of the stable isotopes, ¹²C or ¹³C, of theelement carbon; ²⁶Al relative to the concentration of the stableisotope, ²⁷Al, of the element aluminum) as time passes without suchexposure. Thus, a measure of a concentration of radioactive isotopes ina substance, such as subterranean water, may also indicate an amount oftime that has passed since the substance was last exposed to theatmosphere 101 or surface water.

Turning now to FIG. 3, a flowchart that illustrates an example process300 for testing subterranean water for a ratio of a radioactive isotopeof a particular element with respect to a stable isotope of theparticular element is shown. Process 300 may be performed with thesystem 100. Process 300 begins at step 302, which includes forming atest drillhole from a terranean surface to a subterranean formation. Forexample, the test drillhole 104 may be drilled or otherwise formed fromthe terranean formation to one or both of the subterranean formations108 or 110. Test drillhole 104, may be relatively smaller (e.g., indiameter) than a wellbore formed for the purpose of producinghydrocarbons. Alternatively, test drillhole 104 may be similar to awellbore formed for the purpose of producing hydrocarbon and, in someaspects, may have had hydrocarbons produced therefrom previous to step302.

Process 300 continues at step 304, which includes collecting asubterranean water sample from the subterranean formation. In someaspects, subterranean water may be naturally collected in a downhole endof the drillhole 104 due to, e.g., a pressure difference between one orboth of subterranean formation 108 or 110 and the test drillhole 104(i.e., the formations at a higher fluid pressure than the test drillhole104). In some aspects, the downhole tool 112, e.g., a core drill, may beoperated to obtain core sample 114 which includes subterranean water. Insome aspects, the core sample 114 may have been previously obtained—or asubterranean water sample may have been previously collected from one orboth of formations 108 or 110—prior to the initiation of process 300(e.g., eternal to system 100). Thus, “collecting” in step 304 mayinclude or mean identifying a previously gathered subterranean watersample.

Process 300 continues at step 306, which includes collecting a surfacewater sample from a surface water source. For example, surface water maybe collected from one or both of open water sources 116 or aquifers 118.A surface water sample may also be collected from core sample 120 insurface water formation 106. In some aspects, a surface water sample mayhave been previously collected prior to the initiation of process 300(e.g., eternal to system 100). Thus, “collecting” in step 306 mayinclude or mean identifying a previously gathered subterranean watersample, or identifying a previously determined value of theconcentration of the radioactive isotope relative to the stable isotopeof the element in the surface water. In some aspects, the surface watersample can be sampled from a surface water source. Alternatively, theconcentration of the radioactive isotope (such as ¹²⁹I) compared to thatof the stable isotope (for this case, ¹²⁷I) can be determined from priormeasurements) e.g., prior to the execution of process 300) of theseratios taken from surface water.

Process 300 continues at step 308, which includes determining, with anAMS system, a concentration of a radioactive isotope in the subterraneanwater sample. For example, AMS system 122 may be operated to determine aconcentration of a particular radioactive isotope, such as ³⁶Cl or ¹²⁹I(or both), in the subterranean water sample (e.g., relative to acorresponding stable isotope of that element). In some aspects, step 308may also include determining the concentration of the particularradioactive isotope in the surface water sample as well. Alternatively,the concentration of the radioactive isotope in the surface water samplemay be known (e.g., previous to initiation of the process 300). In someexamples, the determination of the concentration of the radioactiveisotope with the AMS system includes measuring a ratio of theradioactive isotope (e.g., ³⁶Cl or ¹²⁹I) in the particular water sampleto a stable (non-radioactive) isotope (e.g., ³⁵Cl or ¹²⁷I) of the sameelement (chlorine or iodine, respectively). Thus, reference todetermining a concentration of the radioactive element means, in someaspects, determining a ratio of the radioactive isotope to the stable(non-radioactive) isotope of the same element in the particular (surfaceor subterranean, or both) sample.

Process 300 continues at step 310, which includes comparing theconcentrations of the radioactive isotope in the subterranean watersample and the surface water sample. For example, once theconcentrations of the particular isotope or isotopes are determined insurface and subterranean water samples (e.g., relative to acorresponding stable isotope of that element), they are compared todetermine a difference between the two concentrations. For example,generally, the concentrations of ³⁶Cl and ¹²⁹I are higher in the surfacewater sample than the subterranean water sample due the atmosphere 101that was in more recent contact with the surface water sample comparedto the subterranean water sample.

Process 300 continues at step 312, which includes, based on thecomparison, determining that the subterranean formation is suitable as ahazardous waste storage repository. For example, criteria fordetermining that the subterranean formation (108 or 110 or both) issuitable for the long-term (e.g., 100, 1000, 10,000 years or more)storage of hazardous waste material (e.g., spent nuclear fuel) may bethe presence of water that has not been exposed to the atmosphere 101for a particular duration of time, thereby evidencing the subterraneanformation as a geologic formation which does not permit mobile watertherethrough, or otherwise allow a flow of liquid from the formationtoward the surface water formation 106. Such evidence may be proof ofthe subterranean formation to store hazardous waste material with littleto no chance of such material mixing or polluting potable water fit forhuman consumption at the surface water formation 106.

In the example of ³⁶Cl, this radioactive isotope has a half-life of310,000 years. Due to the exposure to the atmosphere 101, ³⁶Cl isproduced in the surface water sample. Subterranean water, however, isnot in contact with the atmosphere 101 and therefore, any ³⁶Cl in thesubterranean water sample decays (e.g., from the moment the water is nolonger in contact with the atmosphere 101. After one half-life, half ofthe ³⁶Cl has decayed from (and is no longer in) the subterranean watersample. After two half-lives, half of the remaining ³⁶Cl has decayedfrom (and is no longer in) the subterranean water sample.

Therefore, in some examples, the subterranean formation (108, 110, orboth) may be suitable as a hazardous waste material repository based onthe concentration of the particular radioactive isotope (e.g., ³⁶Cl or¹²⁹I) in the subterranean water sample being a specified percentage ofthe concentration of the particular radioactive isotope (e.g., ³⁶Cl or¹²⁹I) in the surface water sample. The specified percentage, in someexamples, is between 10 and 50%. Using ³⁶Cl as an example, for 50% asthe specified percentage of the concentration in the subterranean watersample, the time duration since the subterranean water sample last wasexposed to the atmosphere 101 is about 310,000 years (i.e., onehalf-life). For 25% as the specified percentage of the concentration inthe subterranean water sample, the time duration since the subterraneanwater sample last was exposed to the atmosphere 101 is about 620,000years (i.e., two half-lives). For 12.5% as the specified percentage ofthe concentration in the subterranean water sample, the time durationsince the subterranean water sample last was exposed to the atmosphere101 is about 930,000 years (i.e., three half-lives). For 10.6% as thespecified percentage of the concentration in the subterranean watersample, then the time duration since the subterranean water sample lastwas exposed to the atmosphere 101 is about 1,000,000 years. Thus, thespecified percentage may be set to a particular value based on a desiredamount of time that represents the time duration since the subterraneanwater sample last was exposed to the atmosphere 101. If the specifiedpercentage is met, then the subterranean formation from which thesubterranean water sample was obtained may be determined to be asuitable hazardous waste material repository.

Process 300 continues at step 314, which includes creating the hazardouswaste storage repository in or under the subterranean formation. Forexample, a hazardous waste repository may be created as shown in FIG. 2Afor example, which illustrates an example implementation of a hazardouswaste material repository 200.

Process 300 continues at step 316, which includes storing hazardouswaste material in the hazardous waste storage area. FIG. 4 illustratesan example sub-process for step 316 of process 300.

In some implementations, one or both of the subterranean formations 108or 110 may not allow mobile water to pass therethrough or may allowmobile water to pass therethrough only at extremely low velocities.Thus, relative to the surface water formation 106, one or both of thesubterranean formations 108 or 110 may have low permeability, e.g., onthe order of nanodarcy permeability. Additionally, one or both of thesubterranean formations 108 or 110 may be a relatively non-ductile(i.e., brittle) geologic formation. One measure of non-ductility isbrittleness, which is the ratio of compressive stress to tensilestrength. In some examples, the brittleness of the one or both of thesubterranean formations 108 or 110 may be between about 20 MPa and 40MPa.

In some examples, rock formations of which one or both of thesubterranean formations 108 or 110 may be composed include, for example,certain kinds of sandstone, mudstone, clay, and slate that exhibitpermeability and brittleness properties as described above. In someaspects, one or both of the subterranean formations 108 or 110 may bethick, e.g., between about 200 and 2000 feet of total verticalthickness. Thickness of one or both of the subterranean formations 108or 110 may allow for easier landing and directional drilling, therebyallowing a hazardous material storage area to be readily emplaced withinone or both of the subterranean formations 108 or 110 duringconstruction (e.g., drilling). If formed through an approximatehorizontal center of a particular one of the subterranean formations 108or 110, a hazardous waste material storage area may be surrounded byabout 50 to 200 feet of geologic formation.

In some aspects, one or both of the subterranean formations 108 or 110may also have only immobile water, e.g., due to a very low permeability(e.g., on the order of micro- or nanodarcys). In addition, one or bothof the subterranean formations 108 or 110 may have sufficient ductility,such that a brittleness of the rock formation is between about 3 MPa and10 MPa. Examples of rock formations of which one or both of thesubterranean formations 108 or 110 may be composed include shale andanhydrite. Further, in some aspects, hazardous waste material may bestored below one or both of the subterranean formations 108 or 110, evenin a permeable formation such as sandstone or limestone, if one of thesubterranean formations 108 or 110 is of sufficient geologic propertiesto isolate the permeable layer from the surface water formation 106.

In some examples implementations, one or both of the subterraneanformations 108 or 110 is composed of shale. Shale, in some examples, mayhave properties that fit within those described above. For example,shale formations may be suitable for a long-term confinement ofhazardous waste material, and for their isolation from surface waterformation 106 and the terranean surface 102. Shale formations may befound relatively deep in the earth, typically 3000 feet or greater, andplaced in isolation below any fresh water aquifers. Other formations mayinclude salt or other impermeable formation layer.

Shale formations (or salt or other impermeable formation layers), forinstance, may include geologic properties that enhance the long-term(e.g., thousands of years) isolation of material. Such properties, forinstance, have been illustrated through the long-term storage (e.g.,tens of millions of years) of hydrocarbon fluids (e.g., gas, liquid,mixed phase fluid) without escape of substantial fractions of suchfluids into surrounding layers (e.g., surface water formation 106).Indeed, shale has been shown to hold natural gas for millions of yearsor more, giving it a proven capability for long-term storage ofhazardous waste material. Example shale formations (e.g., Marcellus,Eagle Ford, Barnett, and otherwise) has stratification that containsmany redundant sealing layers that have been effective in preventingmovement of water, oil, and gas for millions of years, lacks mobilewater, and can be expected (e.g., based on geological considerations) toseal hazardous waste material (e.g., fluids or solids) for thousands ofyears after deposit.

In some aspects, the formation of one or both of the subterraneanformations 108 or 110 may form a leakage barrier, or barrier layer tofluid leakage that may be determined, at least in part, by the evidenceof the storage capacity of the layer for hydrocarbons or other fluids(e.g., carbon dioxide) for hundreds of years, thousands of years, tensof thousands of years, hundreds of thousands of years, or even millionsof years. For example, one or both of the subterranean formations 108 or110 may be defined by a time constant for leakage of the hazardous wastematerial more than 10,000 years (such as between about 10,000 years and1,000,000 years) based on such evidence of hydrocarbon or other fluidstorage.

Shale (or salt or other impermeable layer) formations may also be at asuitable depth, e.g., between 3000 and 12,000 feet true vertical depth(TVD). Such depths are typically below ground water aquifer (e.g.,surface water formation 106). Further, the presence of soluble elementsin shale, including salt, and the absence of these same elements inaquifer layers, demonstrates a fluid isolation between shale and theaquifer layers.

Another particular quality of shale that may advantageously lend itselfto hazardous waste material storage is its clay content, which, in someaspects, provides a measure of ductility greater than that found inother, impermeable rock formations. For example, shale may bestratified, made up of thinly alternating layers of clays (e.g., betweenabout 20-40% clay by volume) and other minerals. Such a composition maymake shale less brittle and, thus less susceptible to fracturing (e.g.,naturally or otherwise) as compared to rock formations in theimpermeable layer (e.g., dolomite or otherwise). For example, rockformations in the impermeable layer 108 may have suitable permeabilityfor the long-term storage of hazardous waste material, but are toobrittle and commonly are fractured. Thus, such formations may not havesufficient sealing qualities (as evidenced through their geologicproperties) for the long-term storage of hazardous waste material.

The present disclosure contemplates that there may be many other layersbetween or among the illustrated subterranean layers 106, 108, and 110.For example, there may be repeating patterns (e.g., vertically), of oneor more of the surface water formation 106 or one or both of thesubterranean formations 108 or 110. Further, in some instances, thesubterranean formation 108 may be directly adjacent (e.g., vertically)the surface water formation 106, i.e., without an intervening layer. Insome examples, all or portions of the curved drillhole 208 and thestorage drillhole 210 may be formed below one or both of thesubterranean formations 108 or 110, such that the formation in which thesubterranean water has been tested is vertically positioned between thestorage drillhole 210 and the surface water formation 106.

In example implementations, one or both of the subterranean formations108 or 110 may include a self-healing layer. In some aspects, aself-healing layer may comprise a geologic formation that can stop orimpede a flow of hazardous waste material (whether in liquid, solid, orgaseous form) from a storage portion of the drillhole 204 to or towardthe terranean surface 102. For example, during formation of thedrillhole 204 (e.g., drilling), all or portions of the geologicformations 108 or 110, may be disturbed, thereby affecting or changingtheir geologic characteristics (e.g., permeability). A disturbed zonemay surround an entire length (vertical, curved, and inclined portions)of the drillhole 204 a particular distance into the formations 108 and110.

In certain aspects, the location of the drillhole 204 may be selected soas to be formed through a self-healing layer. For example, as shown, thedrillhole 204 may be formed such that at least a portion of the accessportion 206 of the drillhole 204 is formed to pass through theself-healing layer. In some aspects, the self-healing layer comprises ageologic formation that that does not sustain cracks for extended timedurations even after being drilled therethrough. Examples of thegeologic formation in the self-healing layer include shale with highclay content, dolomite, or salt. Cracks in such rock formations tend toheal, that is, they disappear rapidly with time due to the relativeductility of the material, and the enormous pressures that occurunderground from the weight of the overlying rock on the formation inthe self-healing layer. In addition to providing a “healing mechanism”for cracks that occur due to the formation of the drillhole 204 (e.g.,drilling or otherwise), the self-healing layer may also provide abarrier to natural faults and other cracks that otherwise could providea pathway for hazardous waste material leakage (e.g., fluid or solid)from the storage drillhole 210 to the terranean surface 102, the surfacewater formation 106, or both.

FIG. 2A is a schematic illustration of example implementations of ahazardous waste material storage repository, e.g., a subterraneanlocation for the long-term (e.g., tens, hundreds, or thousands of yearsor more) but retrievable safe and secure storage of hazardous wastematerial, during a deposit or retrieval operation according to thepresent disclosure. The hazardous waste material storage repository maybe formed and operated, for example, subsequent to a determination thatone or both of the subterranean formations 108 or 110 are suitable basedon the radioactive isotope testing of the subterranean water asdescribed with reference to FIGS. 1 and 3.

Turning to FIG. 2A, this figure illustrates an example hazardous wastematerial storage repository system 200 during a deposit (or retrieval,as described below) process, e.g., during deployment of one or morecanisters of hazardous waste material in a subterranean formation. Asillustrated, the hazardous waste material storage repository system 200includes a drillhole 204 formed (e.g., drilled or otherwise) from theterranean surface 102 and through the subterranean layers 106, 108, and110. In some aspects, the drillhole 204 may be the same as testdrillhole 104 shown in FIG. 1. Alternatively, drillhole 204 may be anenlarged (e.g., reamed or re-drilled) version of test drillhole 104.Alternatively, the drillhole 204 may be a separate drillhole formedthrough the subterranean layers 106, 108, and into 110.

The illustrated drillhole 204 is a directional drillhole in this exampleof hazardous waste material storage repository system 200. For instance,the drillhole 204 includes an access drillhole 206 coupled to aradiussed or curved portion 208, which in turn is coupled to storagedrillhole 210. In this example, the storage drillhole 210 is horizontal.Alternatively, curved portion 208 may be eliminated and storagedrillhole 210 may be a vertical drillhole that couples to verticalaccess drillhole 204 to forma continuous, vertical drillhole.Alternatively, the curved portion 208 may differ from a 90-degree changein direction, in which case the storage drillhole 210 might be tilted.

The illustrated drillhole 204, in this example, has a surface casing 220positioned and set around the drillhole 204 from the terranean surface102 into a particular depth in the earth. For example, the surfacecasing 220 may be a relatively large-diameter tubular member (or stringof members) set (e.g., cemented) around the drillhole 204 in a shallowformation. As used herein, “tubular” may refer to a member that has acircular cross-section, elliptical cross-section, or other shapedcross-section. For example, in this implementation of the hazardouswaste material storage repository system 200, the surface casing 220extends from the terranean surface through a surface layer 106. In someaspects, the surface casing 220 may isolate the drillhole 204 fromsurface water sources, and may also provide a hanging location for othercasing strings to be installed in the drillhole 204.

As illustrated, a production casing 222 is positioned and set within thedrillhole 204 downhole of the surface casing 220. Although termed a“production” casing, in this example, the casing 222 may or may not havebeen subject to hydrocarbon production operations. Thus, the casing 222refers to and includes any form of tubular member that is placed in thedrillhole 204 downhole of the surface casing 220. In some examples ofthe hazardous waste material storage repository system 200, theproduction casing 222 may begin at an end of the radiussed portion 108and extend throughout the inclined portion 110. The casing 222 couldalso extend into the radiussed portion 108 and into the vertical portion106.

As shown, cement 230 is positioned (e.g., pumped) around the casings 220and 222 in an annulus between the casings 220 and 222 and the drillhole204. The cement 230, for example, may secure the casings 220 and 222(and any other casings or liners of the drillhole 204) through thesubterranean formations under the terranean surface 102. In someaspects, the cement 230 may be installed along the entire length of thecasings (e.g., casings 220 and 222 and any other casings), or the cement230 could be used along certain portions of the casings if adequate fora particular drillhole 204. In some aspects the cement may be omittedaltogether. The cement 230, if used, can also provide an additionallayer of confinement for the hazardous waste material in canisters 226.

The storage drillhole portion 210 of the drillhole 204 includes astorage area in a distal part of the portion 210 into which hazardouswaste material may be retrievably placed for long-term storage. Forexample, as shown, a work string 224 (e.g., tubing, coiled tubing,wireline, or otherwise) may be extended into the cased drillhole 204 toplace one or more (three shown but there may be more or less) hazardouswaste material canisters 226 into long-term, but in some aspects,retrievable, storage in the portion 210. For example, in theimplementation shown in FIG. 2A, the work string 224 may include adownhole tool 228 that couples to the canister 226, and with each tripinto the drillhole 204, the downhole tool 228 may deposit a particularhazardous waste material canister 226 in the storage drillhole portion210.

The downhole tool 228 may couple to the canister 226 by, in someaspects, a threaded connection or other type of connection, such as alatched connection. In alternative aspects, the downhole tool 228 maycouple to the canister 226 with an interlocking latch, such thatrotation (or linear movement or electric or hydraulic switches) of thedownhole tool 228 may latch to (or unlatch from) the canister 226. Inalternative aspects, the downhole tool 228 may include one or moremagnets (e.g., rare earth magnets, electromagnets, a combinationthereof, or otherwise) which attractingly couple to the canister 226. Insome examples, the canister 226 may also include one or more magnets(e.g., rare earth magnets, electromagnets, a combination thereof, orotherwise) of an opposite polarity as the magnets on the downhole tool228. In some examples, the canister 226 may be made from or include aferrous or other material attractable to the magnets of the downholetool 228. Alternative techniques for moving the canisters 226 may alsobe used.

FIG. 2A also illustrates an example of a retrieval operation ofhazardous waste material in the storage drillhole portion 210 of thedrillhole 204. A retrieval operation may be the opposite of a depositoperation, such that the downhole tool 228 (e.g., a fishing tool) may berun into the drillhole 204, coupled to the last-deposited canister 226(e.g., threadingly, latched, by magnet, or otherwise), and pull thecanister 226 to the terranean surface 102. Multiple retrieval trips maybe made by the downhole tool 228 in order to retrieve multiple canistersfrom the storage drillhole portion 210 of the drillhole 204.

Each canister 226 may enclose hazardous waste material. Such hazardouswaste material, in some examples, may be biological or chemical waste orother biological or chemical hazardous waste material. In some examples,the hazardous waste material may include nuclear material, such as spentnuclear fuel recovered from a nuclear reactor (e.g., commercial power ortest reactor) or defense nuclear material. For example, a gigawattnuclear plant may produce 30 tons of spent nuclear fuel per year. Thedensity of that fuel is typically close to 10 (10 gm/cm³=10 kg/liter),so that the volume for a year of nuclear waste is about 3 m³. Spentnuclear fuel, in the form of nuclear fuel pellets, may be taken from thereactor and not modified. Nuclear fuel pellet are solid, although theycan contain and emit a variety of radioactive gases including tritium(13 year half-life), krypton-85 (10.8 year half-life), and carbondioxide containing C-14 (5730 year half-life).

In some aspects, one or both of the subterranean formations 108 or 110may contain any radioactive output (e.g., gases) therewithin, even ifsuch output escapes the canisters 226. For example, one or both of thesubterranean formations 108 or 110 may be shown to contain radioactiveoutput based on the test results of the subterranean water testing asdescribed with reference to FIGS. 1 and 3.

Other criteria in addition to the subterranean water testing asdescribed herein may be used to determine that the subterraneanformations 108 or 110 contain any radioactive output (e.g., gases)therewithin. For example, one or both of the subterranean formations 108or 110 may be selected based on diffusion times of radioactive outputthrough the formations 108 or 110. For example, a minimum diffusion timeof radioactive output escaping the subterranean formations 108 or 110may be set at, for example, fifty times a half-life for any particularcomponent of the nuclear fuel pellets. Fifty half-lives as a minimumdiffusion time would reduce an amount of radioactive output by a factorof 1×10¹⁵. As another example, setting a minimum diffusion time tothirty half-lives would reduce an amount of radioactive output by afactor of one billion.

For example, plutonium-239 is often considered a dangerous waste productin spent nuclear fuel because of its long half-life of 24,200 years. Forthis isotope, 50 half-lives would be 1.2 million years. Plutonium-239has low solubility in water, is not volatile, and as a solid, itsdiffusion time is exceedingly small (e.g., many millions of years)through a matrix of the rock formation that comprise the illustratedsubterranean formations 108 or 110 (e.g., shale or other formation). Thesubterranean formations 108 or 110, for example comprised of shale, mayoffer the capability to have such isolation times (e.g., millions ofyears) as shown by the geological history of containing gaseoushydrocarbons (e.g., methane and otherwise) for several million years. Incontrast, in conventional nuclear material storage methods, there was adanger that some plutonium might dissolve in a layer that comprisedmobile ground water upon confinement escape.

In some aspects, the drillhole 204 may be formed for the primary purposeof long-term storage of hazardous waste materials. In alternativeaspects, the drillhole 204 may have been previously formed for theprimary purpose of hydrocarbon production (e.g., oil, gas). For example,one or both of subterranean formations 108 or 110 may be a hydrocarbonbearing formation from which hydrocarbons were produced into thedrillhole 204 and to the terranean surface 102. In some aspects, thesubterranean formations 108 or 110 may have been hydraulically fracturedprior to hydrocarbon production. Further in some aspects, the productioncasing 222 may have been perforated prior to hydraulic fracturing. Insuch aspects, the production casing 222 may be patched (e.g., cemented)to repair any holes made from the perforating process prior to a depositoperation of hazardous waste material. In addition, any cracks oropenings in the cement between the casing and the drillhole can also befilled at that time.

For example, in the case of spent nuclear fuel as a hazardous wastematerial, the drillhole may be formed at a particular location, e.g.,near a nuclear power plant, as a new drillhole provided that thelocation also includes an appropriate subterranean formation 108 or 110,such as a shale formation. Alternatively, an existing well that hasalready produced shale gas, or one that was abandoned as “dry,” (e.g.,with sufficiently low organics that the gas in place is too low forcommercial development), may be selected as the drillhole 204. In someaspects, prior hydraulic fracturing of the subterranean formations 108or 110 through the drillhole 204 may make little difference in thehazardous waste material storage capability of the drillhole 204. Butsuch a prior activity may also confirm the ability of one or both of thesubterranean formations 108 or 110 to store gases and other fluids formillions of years. If, therefore, the hazardous waste material or outputof the hazardous waste material (e.g., radioactive gasses or otherwise)were to escape from the canister 226 and enter the fractured formationof the subterranean formations 108 or 110, such fractures may allow thatmaterial to spread relatively rapidly over a distance comparable in sizeto that of the fractures. In some aspects, the drillhole 204 may havebeen drilled for a production of hydrocarbons, but production of suchhydrocarbons had failed, e.g., because one or both of the subterraneanformations 108 or 110 comprised a rock formation (e.g., shale orotherwise) that was too ductile and difficult to fracture forproduction, but was advantageously ductile for the long-term storage ofhazardous waste material.

FIG. 2B is a schematic illustration of an example implementation of thehazardous waste material storage repository 200 during storage andmonitoring operations according to the present disclosure. For example,FIG. 2B illustrates the hazardous waste material storage repository 200in a long-term storage operation. One or more hazardous waste materialcanisters 226 are positioned in the storage drillhole portion 210 of thedrillhole 204. A seal 234 is placed in the drillhole 204 between thelocation of the canisters 226 in the storage drillhole portion 210 andan opening of the access drillhole 206 at the terranean surface 102(e.g., a well head). In this example, the seal 234 is placed at anuphole end of the curved portion 108. Alternatively, the seal 234 may bepositioned at another location within the access drillhole 206, in thecurved portion 208, or even within the storage drillhole portion 210uphole of the canisters 226. In some aspects, the seal 234 may be placedat least deeper than any source of surface water, such as the surfacewater formation 106. In some aspects, the seal 234 may be formedsubstantially along an entire length of the access drillhole 206.

As illustrated, the seal 234 fluidly isolates the volume of the storagedrillhole 110 that stores the canisters 226 from the opening of theaccess drillhole 206 at the terranean surface 102. Thus, any hazardouswaste material (e.g., radioactive material) that does escape thecanisters 226 may be sealed (e.g., such that liquid, gas, or solidhazardous waste material) does not escape the drillhole 104. The seal234, in some aspects, may be a cement plug or other plug, that ispositioned or formed in the drillhole 204. As another example, the seal234 may be formed from one or more inflatable or otherwise expandablepackers positioned in the drillhole 204. As another example, the seal234 may be formed of a combination of rock and bentonite. As anotherexample, the seal 234 may be formed from rock similar in composition tothe rock found in nearby layers, such as clay-rich shale.

Prior to a retrieval operation (e.g., as discussed with reference toFIG. 2A), the seal 234 may be removed. For example, in the case of acement or other permanently set seal 234, the seal 234 may be drilledthrough or otherwise milled away. In the case of semi-permanent orremovable seals, such as packers, the seal 234 may be removed from thedrillhole 204 through a conventional process as is known.

Monitoring operations may be performed during long-term storage of thecanisters 226. For example, in some aspects, it may be advantageous orrequired to monitor one or more variables during long-term storage ofthe hazardous waste material in the canisters 226. In an example, amonitoring system includes one or more sensors placed in the drillhole204 (e.g., within the storage drillhole 210) and communicably coupled toa monitoring control system through a cable (e.g., electrical, optical,hydraulic, or otherwise) or through a non-cable method (e.g. acousticsignals). The sensors may be placed outside of the casings, or evenbuilt into the casings before the casings are installed in the drillhole204. Sensors could also be placed outside the casing.

The sensors may monitor one or more variables, such as, for example,radiation levels, temperature, pressure, presence of oxygen, a presenceof water vapor, a presence of liquid water, acidity, seismic activity,or a combination thereof. Data values related to such variables may betransmitted along the cable to the monitoring control system. Themonitoring control system, in turn, may record the data, determinetrends in the data (e.g., rise of temperature, rise of radioactivelevels), send data to other monitoring locations, such as nationalsecurity or environmental center locations, and may furtherautomatically recommend actions (e.g., retrieval of the canisters 226)based on such data or trends. For example, a rise in temperature orradioactive level in the drillhole 204 above a particular thresholdlevel may trigger a retrieval recommendation, e.g., to ensure that thecanisters 226 are not leaking radioactive material. In some aspects,there may be a one-to-one ratio of sensors to canisters 226. Inalternative aspects, there may be multiple sensors per canister 226, orthere may be fewer.

FIG. 4 is a flowchart that illustrates an example implementation of step316 for storing hazardous waste material in a subterranean formationfrom which water has been tested for a radioactive isotope concentrationpercentage. Step 316 may begin with sub-step 402, which includes whichincludes moving a storage canister through an entry of a drillhole thatextends into a terranean surface. The storage canister encloses ahazardous waste material, such as chemical, biological, or nuclearwaste, or another hazardous waste material. In some aspects, the storagecanister may be positioned in the entry directly from a mode oftransportation (e.g., truck, train, rail, or otherwise) which broughtthe hazardous waste material to the site of the drillhole. In someaspects, a packaging of the hazardous waste material during transport isnot removed for movement of the storage canister into the entry. In someaspects, such transport packaging is only removed as the storagecanister fully enters the drillhole.

Step 316 may continue at sub-step 404, which includes moving the storagecanister through the drillhole that includes a substantially verticalportion, a transition portion, and a substantially horizontal portion.In some aspects, the drillhole is a directional, or slant drillhole. Thestorage canister may be moved through the drillhole in a variety ofmanners. For example, a tool string (e.g., tubular work string) orwireline may include a downhole tool that couples to the storagecanister and moves (e.g., pushes) the storage canister from the entry tothe horizontal portion of the drillhole. As another example, the storagecanister may ride on rails installed in the drillhole, e.g., a caseddrillhole. As yet another example, the storage canister may be movedthrough the drillhole with a drillhole tractor (e.g., motored or poweredtractor). In another example, the tractor could be built as part of thestorage canister. As yet a further example, the storage canister may bemoved through the drillhole with a fluid (e.g., gas or liquid)circulated through the drillhole.

Step 316 may continue at sub-step 406, which includes moving the storagecanister into a storage area located within or below a shale formation.For example, the horizontal portion of the drillhole may include or becoupled to the storage area and may be formed through a shale seamwithin a subterranean zone. In some aspects, the shale may include oneor more geologic qualities that provide for a fluidic seal (e.g., gasand liquid) against the escape of any hazardous waste material beyondthe shale formation (e.g., vertically or horizontally). In alternativeaspects, the storage area may be formed in the horizontal portion of thedrillhole in a rock formation that is not shale, but shares particulargeologic characteristics with shale (e.g., anhydrite, and otherformations). For example, the rock formation of the storage area may berelatively impermeable, with permeability values less than 0.001millidarcys (and even down to nanodarcys). As another example, the rockformation may be ductile, having a brittleness of less than about 10 MPaso as to prevent or help prevent fracturing that can allow hazardouswaste material leaks therethrough. Brittleness, as used herein inexample implementations, is the ratio of compressive stress of the rockformation to tensile strength of the rock formation. As another example,the rock formation may be relatively thick, with thickness proximate thestorage area of between about 100 and 200 feet (although less thick andmore thick formations are also contemplated by the present disclosure).As another example, the rock formation may be composed of clay or otherorganic material, e.g., of about 20-40% weight by volume, to helpductility. As another example, the rock formation may be composed ofsalt.

Step 316 may continue at sub-step 408, which includes forming a seal inthe drillhole that isolates the storage portion of the drillhole fromthe entry of the drillhole. For example, once the storage canister ismoved into the storage area (or after all storage canisters are movedinto the storage area), a seal may be formed in the drillhole. The sealmay be a cement plug, an inflatable seal (e.g., packer), a regioncontaining a mixture of rock and bentonite, or other seal or combinationof such seals. In some aspects, the seal is removable so as tofacilitate a subsequent retrieval operation of the storage canister.

Step 316 may continue at sub-step 410, which includes monitoring atleast one variable associated with the storage canister from a sensorpositioned proximate the storage area. The variable may include one ormore of temperature, radioactivity, seismic activity, oxygen, watervapor, acidity, or other variable that indicates a presence of thehazardous waste material (e.g., within the drillhole, outside of thestorage canister, in the rock formation, or otherwise). In some aspects,one or more sensors may be positioned in the drillhole, on or attachedto the storage canister, within a casing installed in the drillhole, orin the rock formation proximate the drillhole. The sensors, in someaspects, may also be installed in a separate drillhole (e.g., anotherhorizontal or vertical drillhole) apart from the storage area.

Step 316 may continue at sub-step 412, which includes recording themonitored variable at the terranean surface. For example, variable datareceived at the one or more sensors may be transmitted (e.g., on aconductor or wirelessly) to a monitoring system at the terraneansurface. The monitoring system may perform a variety of operations. Forexample, the monitoring system may record a history of one or more ofthe monitored variables. The monitoring system may provide trendanalysis in the recorded variable data. As another example, themonitoring system may include one or more threshold limits for each ofthe monitored variables, and provide an indication when such thresholdlimits are exceeded.

Step 316 may continue at sub-step 414, which includes determiningwhether the monitored variable exceeds a threshold value. For example,the one or more sensors may monitor radioactivity in the drillhole,e.g., an amount of radiation emitted by the hazardous waste material,whether in alpha or beta particles, gamma rays, x-rays, or neutrons. Thesensors, for instance, may determine an amount of radioactivity, inunits of measure of curie (Ci) and/or becquerel (Bq), rads, grays (Gy),or other units of radiation. If the amount of radioactivity does notexceed a threshold value that, for example, would indicate a large leakof hazardous nuclear material from the storage canister, then the step316 may return to sub-step 410.

If the determination is “yes,” sub-step 316 may continue at sub-step416, which includes removing the seal from the drillhole. For example,in some aspects, once a threshold value (or values) is exceeded, aretrieval operation may be initiated by removing the seal. Inalternative aspects, exceeding of a threshold value may notautomatically trigger a retrieval operation or removal of the drillholeseal. In some aspects, there may be multiple monitored variables, and a“yes” determination is only made if all monitored variables exceed theirrespective threshold values. Alternatively, a “yes” determination may bemade if at least one monitored variable exceeds its respective thresholdvalue.

Step 316 may continue at sub-step 418, which includes retrieving thestorage canister from the storage area to the terranean surface. Forexample, once the seal is removed (e.g., drilled through or removed tothe terranean surface), the work string may be tripped into thedrillhole to remove the storage canister for inspection, repair, orotherwise. In some aspects, rather than removing the seal from thedrillhole to retrieve the storage canister, other remedial measures maybe taken. For example, if the determination is “yes” in sub-step 414,rather than recovering the hazardous waste material, a decision might bemade to improve the seal. This could be done, for example, by injectingbentonite, a cement, or other sealant into the borehole to fill thespace previously filled with gas.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, exampleoperations, methods, or processes described herein may include moresteps or fewer steps than those described. Further, the steps in suchexample operations, methods, or processes may be performed in differentsuccessions than that described or illustrated in the figures.Accordingly, other implementations are within the scope of the followingclaims.

What is claimed is:
 1. A method, comprising: collecting, from a testdrillhole formed from a terranean surface to a subterranean formation, asubterranean water sample from the subterranean formation; determining,with an accelerator mass spectrometry (AMS) system, a concentration of aradioactive isotope of an element in the subterranean water samplerelative to a stable isotope of the element in the subterranean watersample; comparing the determined concentration of the radioactiveisotope of the element in the subterranean water sample with aconcentration of the radioactive isotope of the element in a surfacewater sample relative to the stable isotope of the element in thesurface water sample; based on the determined concentration of theradioactive isotope in the subterranean water sample being a specifiedpercentage of the concentration of the radioactive isotope in thesurface water sample, determining that the subterranean formation issuitable as a hazardous waste storage repository; and based on thedetermination that the subterranean formation is suitable as thehazardous waste storage repository, forming an access drillhole from theterranean surface toward the subterranean formation.
 2. The method ofclaim 1, wherein: the radioactive isotope is carbon-14 (¹⁴C) and thestable isotope is ¹²C or ¹³C; the radioactive isotope is chlorine-36(³⁶Cl) and the stable isotope is ³⁵Cl; the radioactive isotope isiodine-129 (¹²⁹I) and the stable isotope is ¹²⁷I; the radioactiveisotope is beryllium-10 (¹⁰Be) and the stable isotope is ⁹Be; or theradioactive isotope is aluminum-26 (²⁶Al) and the stable isotope is²⁷Al.
 3. The method of claim 1, wherein the specified percentage is lessthan 50 percent.
 4. The method of claim 1, further comprising collectingthe surface water sample from a surface water source.
 5. The method ofclaim 4, wherein the surface water source comprises at least one of anaquifer or a water source at the terranean surface in contact with theearth's atmosphere.
 6. The method of claim 1, wherein the surface watersample comprises potable water.
 7. The method of claim 1, whereincollecting the subterranean water sample from the subterranean formationcomprises: operating a downhole tool in the test drillhole to collect acore sample from the subterranean formation; retrieving the core sampleto the terranean surface; and removing water from the core sample, theremoved water comprising the subterranean water sample.
 8. The method ofclaim 4, wherein collecting the surface water sample from the surfacewater source is performed prior in time to collecting the subterraneanwater sample from the subterranean formation.
 9. The method of claim 1,wherein the subterranean formation comprises a shale formation.
 10. Themethod of claim 1, wherein the subterranean formation comprises apermeability of less than about 0.01 millidarcys.
 11. The method ofclaim 1, further comprising forming the test drillhole from theterranean surface to the subterranean formation.
 12. The method of claim1, wherein the test drillhole comprises a vertical drillhole.
 13. Themethod of claim 1, wherein the subterranean formation comprises abrittleness of less than about 10 MPa, where brittleness comprises aratio of compressive stress of the subterranean formation to tensilestrength of the subterranean formation.
 14. The method of claim 1,wherein the subterranean formation comprises about 20 to 40% weight byvolume of clay or organic matter.
 15. The method of claim 1, wherein thesubterranean formation comprises an impermeable layer.
 16. The method ofclaim 1, wherein the subterranean formation comprises a leakage barrierdefined by a time constant for leakage of a hazardous waste material of10,000 years or more.
 17. The method of claim 1, wherein thesubterranean formation comprises a hydrocarbon or carbon dioxide bearingformation.
 18. The method of claim 1, further comprising initiatingcreation of the hazardous waste storage repository in or under thesubterranean formation.
 19. The method of claim 18, wherein initiatingcreation of the hazardous waste storage repository in or under thesubterranean formation comprises: forming a storage drillhole coupled tothe access drillhole in or under the subterranean formation, the storagedrillhole comprising a hazardous waste storage area.
 20. The method ofclaim 19, wherein the access drillhole comprises a vertical drillhole.21. The method of claim 19, wherein the test drillhole comprises aportion of the access drillhole.
 22. The method of claim 19, wherein thestorage drillhole comprises a curved portion and a horizontal portion.23. The method of claim 19, wherein the subterranean formation comprisesa thickness proximate the hazardous waste material storage area of atleast about 200 feet.
 24. The method of claim 19, wherein thesubterranean formation comprises a thickness proximate the hazardouswaste material storage area that inhibits diffusion of a hazardous wastematerial through the subterranean formation for an amount of time thatis based on a half-life of the hazardous waste material.
 25. The methodof claim 19, further comprising installing a casing in the accessdrillhole and the storage drillhole that extends from at or proximatethe terranean surface, through the access drillhole and the storagedrillhole, and into the hazardous waste material storage area of thestorage drillhole.
 26. The method of claim 25, further comprisingcementing the casing to the access drillhole and the storage drillhole.27. The method of claim 19, further comprising, subsequent to formingthe access drillhole, producing hydrocarbon fluid from the subterraneanformation, through the access drillhole, and to the terranean surface.28. The method of claim 19, further comprising storing hazardous wastematerial in the hazardous waste storage area.
 29. The method of claim28, wherein storing hazardous waste material in the hazardous wastestorage area comprises: moving a storage canister through an entry ofthe access drillhole that extends into the terranean surface, the entryat least proximate the terranean surface, the storage canistercomprising an inner cavity sized to enclose the hazardous wastematerial; moving the storage canister through the access drillhole andinto the storage drillhole; and moving the storage canister through thestorage drillhole to the hazardous waste storage area.
 30. The method ofclaim 29, further comprising forming a seal in at least one of theaccess drillhole or the storage drillhole that isolates the hazardouswaste storage area from the entry of the access drillhole.
 31. Themethod of claim 28, wherein the hazardous waste material comprises spentnuclear fuel.
 32. The method of claim 29, wherein the storage canistercomprises a connecting portion configured to couple to at least one of adownhole tool string or another storage canister.
 33. The method ofclaim 29, further comprising monitoring the hazardous waste materialstored in the hazardous waste material storage area of the storagedrillhole.
 34. The method of claim 33, wherein monitoring the hazardouswaste material stored in the hazardous waste material storage area ofthe storage drillhole comprises: removing the seal; and retrieving thestorage canister from the hazardous waste material storage area to theterranean surface.
 35. The method of claim 33, wherein monitoring thehazardous waste material stored in the hazardous waste material storagearea of the storage drillhole comprises: monitoring at least onevariable associated with the storage canister from a sensor positionedproximate the hazardous waste material storage area; and recording themonitored variable at the terranean surface.
 36. The method of claim 35,wherein the monitored variable comprises at least one of radiationlevel, temperature, pressure, presence of oxygen, presence of watervapor, presence of liquid water, acidity, or seismic activity.
 37. Themethod of claim 36, further comprising, based on the monitored variableexceeding a threshold value: removing the seal; and retrieving thestorage canister from the hazardous waste material storage drillholeportion to the terranean surface.